feat: create an agent plugin #52

Which area does this relate to?

• CLI code
• SPEC or COMPLIANCE file
• Other doc files

What kind of feature are you proposing?

New command or flag

What would you like to see?

Agent Plugin for a2a-cli.

Why is this important?

Agent Plugin provide easy to mechanism to integrate to agentic harness like Antigravity, Nanoclaw, VSCode, GitHub & Hermes.

Who benefits from this feature?

Developers.

Would you be interested in helping build this feature?

Yes — I'd like to implement it

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct